1. Field of the Invention
The invention is concerned with a carbon brush used in an electrical machine with commutator for use in a vehicle.
2. Description of Related Art
In accordance with current technology, carbon brushes of this type are constructed to have a specific electrical resistance by suitable selection and dimensioning of the brush material, particularly of the copper component, which takes into account specific requirements depending on the vehicle voltage. During operation of an electrical machine of this kind with commutator, a brush will short circuit two neighbouring commutator segments when passing from one segment to the next neighbouring segment. The short circuit current which results from this, the magnitude of which depends on the overall resistance formed by the resistance of the carbon brush plus the contact resistance between the carbon brush and the collector segment, should be kept to a minimum for good commutation. This requirement would indicate the need for a relatively high specific resistance of the carbon brush material, which, however, is in contradiction to another requirement: for on the other hand, the wiring loss in the electrical machine of electrical power that is transmitted through the carbon brushes should be as small as possible. This problem becomes considerably more acute if an attempt is made to adapt an electrical machine which has been designed for a certain operating voltage to work at a higher operating voltage, but making as few drastic design changes as possible. If a motor is to be dimensioned to operate at a higher voltage while retaining as far as possible the existing collector, an increase is needed in the number of windings, whose inductance then increases in accordance with the square law. There is an additional load placed on the sliding contacts of the commutator owing to the increased voltage.
For this reason, it is usual to increase the number of commutator segments or collector segments when the electrical machine is adapted to operate at a higher voltage. However, this involves a considerably greater amount of work. Motors normally found in vehicle applications have between three and 24 segments.
Carbon brush materials used in practice have the following specific resistance values for the relevant operating voltage of a vehicle electrical system: carbon brushes intended to be used in vehicles with a 12 volt electrical system have a specific resistance of less than 100 .mu..OMEGA.cm if the carbon brush is to be used in a starter motor, in the range 50-1000 .mu..OMEGA.cm for auxiliary motors and alternators, and up to several thousand .mu..OMEGA.cm in the special case of motors used for fuel pumps. For vehicles with a 24 volt electrical system, the specific resistance of the carbon brushes is between 20 and 100 .mu..OMEGA.cm for starter motors, in the range 250-4000 .mu..OMEGA.cm for auxiliary motors, and up to 7000 .mu..OMEGA.cm for the carbon brushes of motors used for fuel pumps. For forklift trucks using a 48 volt electrical system, carbon brush materials with a specific resistance between 500 and 2500 .mu..OMEGA.cm have been successfully used; carbon brush materials with a specific resistance of approximately 10,000 .mu..OMEGA.cm have also been used, but only in individual cases, namely in reversible motors where particularly high demands are placed on the commutators. In completely different areas of application for household equipment and portable power tools which operate at 110 or 230 volts, carbon brushes can be found with a specific resistance between 25,000 and 200,000 .mu..OMEGA.cm.